Caption: Influenza vaccine. Pre-filled syringes containing the influenza (flu) vaccine Invivac. The vaccine contains surface protein subunits from 3 strains of virus embedded in a liposome membrane, known as a virosome. The strains are those thought most likely to cause outbreaks in 2005 and 2006, and are chosen by the World Health Organisation and the European Union. Vaccines provide protection against disease by priming the body's immune system, without causing infection. White blood cells are produced that can fight any future infection with the same or similar strains of flu virus. Due to the virus' ability to produce new strains, a new vaccine is needed each year. Invivac is produced by Solvay Pharmaceuticals.